Method and bath for electrodepositing bright silver



United States Patcnt O 3,215,610 METHOD AND BATH FOR ELECTRO- DEPOSITINGBRIGHT SILVER Willi Skaliks, Berlin-Zehlendorf, Germany, assignor toSchering, A.G., Berlin, Germany No Drawing. Filed May 8, 1962, Ser. No.193,302

Claims priority, application Germany, May 19, 1961, Sch 29,728

14 Claims. (Cl. 204-46) This invention relates to a method of forming abright silver deposit by electrolysis of an aqueous bath, and to a bathsuitable for performing the method.

It is known in the art that additions of small amounts of tetravalent ordivalent selenium to an otherwise conventional alkaline plating bathcontaining a complex silver salt such as silver cyanide improves thebrightness of the electrodeposit formed when the article is to be platedat the cathode in such a bath. However, it has been found that the knownsilver plating baths containing selenium compounds as brighteners arenot entirely satisfactory. The known baths containing quadrivalentselenium have no leveling effect, and actually may enhance pre-existingsurface defects such as grinding marks.

The known divalent selenium brighteners are free from this shortcoming,but the bright plating range available with divalent selenium compoundsas brighteners in the electrodeposition of silver is substantiallylimited to cath ode current densities of 1 ampere per square decimeteror more. Deposits produced at lower current densities have a hazy ormilky appearance.

It has been found that the brightness of silver depositselectrolytically produced in the presence of compounds of selenium,which selenium has the valence of minus 2, can be greatly improved, andthat the bright plating range can be extended to a lower limit which isa small fraction of one ampere per square decimeter, and that othervaluable properties may be imparted to the silver electrodeposit byemploying with the selenium brightener a condensation product ofnaphthalene-alpha-sulfonic acid with formaldehyde. The formula of thisproduct is wherein X is a cation which permits the condensation productto be dissolved in the alkaline electrolyte, and does not give rise toundesirable decomposition products in the bath, and n is an integer from4 to 6.

The compounds of the above formula in which- X is either sodium orpotassium arereadily available, but other alkali metal salts and theammonium salt are equally usable since the cation does not appear toparticipate in the brightening effect if it is stable in the solution.

It has also been found that the afore-described condensation product isan excellent silver brightener when emloyed in the absence of a seleniumcompound. Whether used with or without an auxiliary selenium brightener,the condensation product is employed in concentrations of 0.02 to 0.5gram per liter of the plating bath. The actual concentration chosen willdepend upon the concentration of the other conventional constituents ofthe bath, on the deposition rate, on the result desired, and on othervariables in a manner well known in this art.

The condensation product of naphthalene-alpha-sulfonic acid andformaldehyde is available as an article of 3,215,610 Patented Nov. 2,1965 commerce, and is being sold for example, under the trade names andtrademarks Solegal A, Daxad 11, and Compound 8-Alpha in the UnitedStates and abroad.

Suitable selenium compounds include the selenides of the alkali metalsand of ammonium, selenocyanides, and numerous organic selenium compoundsin which selenium is divalent and negatively charged, and which are wellknown to have some brightening effect when added to complex silver saltsolutions in the absence of the condensation product of naphthalenesulfonic acid and formaldehyde.

The electrolytes of this invention are surprisingly resistant tocontamination by dust and other airborne contaminants. It has also beenfound that excellent bright silver deposits can be obtained fromsolutions used over a long period of time without forming a sludge ofcyanide decomposition products and of other impurities in the bath. Thesilver deposits formed have a distinct leveling effect so that surfaceirregularities of the article coated with the silver deposit tend to beobscured as the thickness of the silver layer increases. Deposits formedin the presence of approximately 0.001 to 0.1 gram per liter of divalentselenium are mirror bright when deposited at current densities rangingat least from 0.1 to 5.0 amperes per square decimeter.

The silver deposit is dense and free from pores even when formed in theabsence of the wetting agents normally employed to prevent gas bubblesfrom adhering to the plated surface. The full brightness of the silverdeposit is apparent as soon as the first visible layer of silver isdeposited. Silver layers as thin as 0.2 micron show uniform mirrorbrightness. Because of the wide bright plating range of the electrolytesof the invention, the control of cathode current density is notcritical, and uniform bright deposits are readily formed on articles ofcomplex shape without the use of auxiliary anodes or similar expedients.

. The deposits have an initial hardness of to kilograms per squaremillimeter on the Vickers scale. Hardness drops after some time toapproximately 90 kilograms per square millimeter. In contrast to othermethods used, this hardness is still unusually high, and makes thesilver deposits of the invention eminently suitable for use on articlessubject to mechanical wear such as cutlery, switches, and otherelectrical contacts.

Also, it is to be noted that the electroplating baths of the inventionare relatively insensitive to variations in op erating conditions.Neither temperature nor current density is particularly critical, andthey do not unfavorably affect the properties of the silver platingproduced when held within reasonable limits readily maintained withoutcomplex controls. .The fact that wetting agents are not necessary tocontrol porosity increases the useful life of the solutions which arenot contaminated by decomposition products of the surface-active agents.Periodic or continuous filtration of the baths and purification withactivated charcoal are not necessary with the silver plating baths ofthe invention.

Silver is commercially deposited from complex silver cyanide solutionsalmost to the exclusion of other complex salts of the metal. In thefollowing illustrative specific examples of the method of the invention,and of the baths employed for performing the method, complex silvercyanide baths therefor have been described. However, it is to beunderstood that the invention is not to be limited to such examples. butthe invention is not limited to any particular complex silver salt, andthose skilled in the art may substitute other alkaline complex silversolutions and employ ade quate voltage to decompose silver complexesother than the cyanide complex.

Such baths are preferred,-

3 Example 1 An electrolyte is prepared by dissolving in one liter ofwater under ordinary conditions, the following:

40.0 grams silver cyanide 130.0 grams potassium cyanide 20.0 gramspotassium carbonate 0.04 gram selenium as K Se 0.07 gram condensationproduct of formaldehyde and naphthalenesulfonic acid in the form of thesodium salt Mirror bright silver deposits are obtained over a currentdensity range from 0.1 to 4.0 amperes per square decimeter when eitherthe electrolyteor the cathode is agitated. The brightness of the depositis virtually unaffected by temperature variations between and 30 C.

Example 2 An electrolyte is prepared from one liter of water underordinary conditions and 30.0 grams silver cyanide 120.0 grams potassiumcyanide 30.0 grams potassium carbonate 0.06 gram selenium as KCNSe 0.10gram condensation product of formaldehyde and naphthalene-alpha-sulfonicacid in the form of the potassium salt Mirror bright silver depositsare. obtained at current densities between 0.1 and 4.0 amperes persquare decimeter and at temperatures between 10 and 30 C. when eitherthe electrolyte is agitated or the article to be plated is suspendedfrom a reciprocating cathode rod in the usual manner.

Example 3 An aqeuous electrolyte is prepared from one liter of water and25.0 grams silver cyanide 130.0 grams potassium cyanide 30.0 gramspotassium carbonate 0.2 gram condensation product of formaldehyde andnaphthalenesulfonic acid in the form of the potassium salt Semi-brightsilver deposits are obtained at cathode current densities of 0.2 to 3.0amperes per square decimeter, and at temperatures between 10 and 30 C.when relative movement of electrolyte and cathode is providedby eitheragitating the bath or moving the article which is being plated.

While the invention has been described with particular reference tospecific embodiments, it is to be understood that it is not limitedthereto, but is to be construed broadly, and is restricted solely by thescope of the appended claims.

I claim:

1. A bath for the electrolytic deposition of silver comprising anaqueous alkaline solution of a complex silver salt and an effectiveamount of a brightener dissolved in said solution and having the formulaSOaX wherein n is an integer from four to six; and X is a cation stablein'said solution.

2. A bath as set forth in claim 1, wherein X is an alkali metal.

3. A bath as set forth in claim 2, wherein X is sodium.

4. A bath as set forth in claim 2, wherein X is potassium.

5. A bath as set forth in claim 1, wherein the concentration of saidbrightener is between substantially 0.02 and 0.5 gram per liter.

6. A bath as set forth in claim 1, further comprising as an auxiliarybrightener a compound of selenium dissolved in said solution, saidselenium compound containing selenium in the divalent and negativelycharged form.

7. A bath as set forth in claim 6, wherein the concentration of saidselenium is substantially between 0.001 and 0.1 gram per liter.

8. Abath as set forth in claim 1 wherein said complex silver salt is acomplex cyanide.

9. A method of electrodepositing silver on a conductive article whichcomprises making said article the cathode in an aqueous alkalinesolution of a complex silver salt containing as a brightener aneffective amount of a compound dissolved in said solution and having theformula SIOQX wherein n is an integer from four to six; and X is acation stable in said solution.

10. A method as set forth in claim 9, wherein X is an alkali metalselected from the group consisting of sodium and potassium.

11. A method as set forth in claim 10, wherein the concentration of saidbrightener is between substantially 0.02 and 0.5 gram per liter.

12. A method as set forth in claim 9, wherein said solution furthercontains as an auxiliary brightener a dissolved compound of selenium,said selenium compound containing selenium in the divalent andnegatively charged form.

13. A method as set forth in claim 12 wherein the concentration of saidselenium in said solution is substantially between 0.001 and 0.1 gramper liter.

14. A method of electrodepositing silver on a conductive article whichcomprises making said article the cathode at a predetermined voltage inan aqueous alkaline solution of a complex silver salt containing as abrightener 0.02 to 0.5 gram per liter of a compound dissolved in saidsolution and having the formula SOaX wherein n is an integer from fourto six, and X is an alkali metal selected from the group consisting ofsodium and potassium and 0.001 and 0.1 gram per liter of selenium in thedivalent and negatively charged form, said selenium being present as acompound of selenium dissolved in said solution, said voltage beingsufficient to deposit silver from saidsolution on said conductivearticle.

References Cited by the Examiner UNITED STATES PATENTS 2,195,409 4/ 40Flett 20446 2,777,810 l/ 57 Ostrow 20446 JOHN H. MACK, Primary Examiner.

JOHN R, SPECK, Examiner.

1. A BATH FOR THE ELECTROLYTIC DEPOSITION OF SILVER COMPRISING ANAQUEOUS ALKALINE SOLUTION OF A COMPLEX SILVER SALT AND AN EFFECTIVEAMOUNT OF A BRIGHTENER DISSOLVED IN SAID SOLUTION AND HAVING THE FORMULA6. A BATH AS SET FORTH IN CLAIM 1, FURTHER COMPRISING AS AN AUXILIARYBRIGHTENER A COMPOUND OF SELENIUM DISSOLVED IN SAID SOLUTION, SAIDSELENIUM COMPOUND CONTAINING SELENIUM IN THE DIVALENT AND NEGATIVELYCHARGED FORM.